Nitrate (NO3) is a nitrogen by-product of the nitrifying bacteria (Nitrospira) in a filter or substrate consuming Nitrite. It was previously thought species of Nitrobacter did this. but since 1998 it's considered to be species of Nitrospira.

Nitrate is far less toxic than nitrite and much less toxic than Ammonia.

The level of Nitrate in water that may damages aquatic animals varies considerably in each species and at their stage of growth. With mortality of eggs and very young fry being sensitive to low levels (~20 mg/l) of nitrate.[1] Also nitrate toxicity is made further complex as fish can become accustomed to a slowly growing level of nitrate over time and remain seemingly visually unharmed.

Scientific studies into nitrate toxicity have been primary been performed on commercial fish consumed by the public. For example fish like Salmon, trout and some large catfish. With the majority of the fish used in the ornamental pet trade remaining untested and therefore an unknown factor.

Studies of adult fish like Salmon show they can tolerate levels of 5,000 mg/l [1][2]

The safe level of nitrate varies considerably between species and its age. The hobby has settled for an average maximum level of 50 mg/l in a typical community tropical tank. But this level may need to be adjusted downwards if you wish to own known nitrate sensitive fish like Stingrays or Discus. Marine fish owners often set a safe level of below 20 mg/l.

It inhibits growth, damages internal organs and impairs the immune system in young fish.
In older fish high levels cause stress leading to a depressed immune system, behavioural changes and even blindness and death.

There is no single level 'dangerous' dosage. Aquarists playing safe by trying to keep this level below 50 mg/l. Though in reality species vary enormously in their tolerance to this toxin. The theory today is that prolonged exposure to elevated levels of nitrate may decrease the immune response, induce internal hematological and biochemical changes within the animal (behaviour changes) and may increase mortality. Especially in fry.[3]

The average fish can withstand quite high dosages of nitrates (100-500mg/l) as long as the build up of the chemical is slow and over many days in the tank. However it's quite common for less experience aquarists to go and introduce a new fish to their seemingly healthy tank of fish only for the new fish to die overnight due to nitrate poisoning if the tank water is heavy with nitrate. And as stated previously each species tolerance is different (and poorly researched in the cases of ornamental aquatic pets) which is why the hobby often recommends an average safe level of 50mg/l.

There are many test kits available from pet shops for testing the level of nitrate in water. Some are simple strips of cards which you dip into water. Other more accurate ones use drops of chemicals you mix with a sample of water.

Aquarists should always have a test kit on hand. If there is an unexplained death or behaviour in the tank then an immediate test of nitrates, nitrites, etc. will be required as soon as possible in case action needs to be carried out.

Dose with fruit sugar fructose (typically 4g per 20 litres of water once a week or half that amount of fructose if nitrates are higher than 100mg/l) which promotes nitrate consuming bacteria by supplying a source of dissolved carbon for the bacteria to consume nitrogen.[4] Ensure aquarium is well aerated during this treatment.

Add a nitrate reactor which removes the nitrate by chemical or bacteria process.

In a well planted tank with a good CO2 supply, the plants will want to consume more nitrate than there is in the water.
In these situations it is often the case that aquatics manually add nitrate via chemical powders.